3.2. Conditionals¶

In order to write useful programs, we almost always need the ability to have the program take different actions based on conditions that are checked when it is running. That is, do one thing if something is true, or do another thing if it is not true. Conditional statements give us this ability. The simplest form is the if statement, which has shown up a few times in examples already:

if x > 0:
    print("x is positive.")

This can be read aloud as: “If x is greater than zero, then print ‘x is positive.’”

The expression after the if statement is called the condition; here it is x > 0. We end the if statement with a colon : and the line(s) after the if statement, the body of the conditional, are indented.

Syntax Pattern

A basic if statement has the form:

if <condition>:
    <body>

Python interprets this as follows:

It evaluates the expression in <condition>.
If the condition evaluates to True, then it executes the statements in <body>.
Otherwise (if the condition is False), then the body is skipped.

Visual flowchart illustrating the execution of a basic if statement. — Flowchart illustrating the execution of a basic if statement.¶

Notice the consistency in the Python syntax here: if statements have the same basic structure as for loops. The statement consists of a header line that ends with the colon : followed by an indented block of statements that make up its body. Statements like this are called compound statements because they stretch across more than one line.

There is no limit on the number of statements that can appear in the body, but there must be at least one. Occasionally, it is useful to have a body with no statements (usually as a place holder for code you haven’t written yet). In that case, you can use the pass statement, which does nothing.

if x < 0:
    pass    # this doesn't do anything (yet)

3.2.1. Alternative execution¶

A second form of the if statement is alternative execution or if-else statement, in which there are two possibilities and the condition determines which one gets executed. The syntax looks like this:

If the remainder when x is divided by 2 is 0, then we know that x is even, and the program displays a message to that effect. If the condition is false, the indented block after the else line is executed.

Syntax Pattern

An if-else statement has the form:

if <condition>:
    <body1>
else:
    <body2>

Python interprets this as follows:

It evaluates the expression in <condition>.
If the condition evaluates to True, then it executes the statements in <body1>.
Otherwise (if the condition is False), then it executes the statements in <body2>.

Visual flowchart illustrating the execution of an if-else statement. — Flowchart illustrating the execution of an if-else statement.¶

Since the condition must either be true or false, exactly one of the alternatives will be executed. The alternatives are called branches, because they are branches in the flow of execution.

3.2.2. Chained conditionals¶

Sometimes there are more than two possibilities and we need more than two branches. One way to express a computation like that is a chained conditional:

elif is an abbreviation of “else if.” Again, exactly one branch will be executed. There is no limit on the number of elif statements. If there is an else clause, it has to be at the end, but there doesn’t have to be one.

if choice == 'a':
    print('Bad guess')
elif choice == 'b':
    print('Good guess')
elif choice == 'c':
    print('Close, but not correct')

Each condition is checked in order. If the first is false, the next is checked, and so on. If one of them is true, the corresponding branch executes, and the statement ends.

Note

Even if more than one condition is true, only the first true branch executes.

Syntax Pattern

A general conditional statement has the form:

if <condition1>:
    <body1>
# There can be one or more elif sections (optional)
elif <condition2>:
    <body2>
# There can be one else section at the end (optional)
else:
    <body3>

Python interprets this as follows:

It evaluates the condition expression in the if statement.
If the condition evaluates to True, then it executes the statements in <body1> and leaves the conditional (skipping all elif and else sections).
Otherwise (if the condition is False), then it evaluates the expression in the next elif statement.
If that condition evaluates to True, then it executes the statements in <body2> and leaves the conditional (skipping all elif and else sections).
Steps 3 and 4 are repeated for each elif statement.
If no condition matches and an else statement is present, it executes the statements in the else block.

Visual flowchart illustrating the execution of a general conditional statement. — Flowchart illustrating the execution of general conditional statement.¶

3.2.3. Nested Blocks¶

Notice how any time we’ve introduced a syntax pattern with an indented <body>, we’ve talked about it as being made up of statements? It’s important to understand that the statements inside the body of a for loop or conditional (or any other indented body) can be any valid Python statements, including for loops or conditionals themselves!

So for example, one conditional can be nested within another, and we could have written the three-branch example like this:

if x < y:
    print(x, 'is less than', y)
else:
    if x > y:
        print(x, 'is greater than', y)
    else:
        print(x, 'and', y, 'are equal')

The outer conditional contains two branches. The first branch contains a simple print statement. The second branch contains another if statement, which has two branches of its own. Those two branches are both simple statements, although they could have been conditional statements as well.

Although the indentation of the statements makes the structure apparent, nested conditionals become difficult to read very quickly. In general, it is a good idea to avoid them if you can. But don’t be afraid to put a for loop inside a conditional, or a conditional inside a for loop. It’s perfectly valid and quite common. You can see an example in the example program in the Introduction.